As typical techniques for attaining synchronization in a LAN network, for example, such as Ethernet or the like, a method (hereinafter, referred to as a Sync-E (Synchronous Ethernet)) which is provided in ITU-T G. 8261 and is of the type that synchronization is attained using a clock which is extracted from a transmission line and a method (hereinafter, referred to as PTP (Precision Time Protocol)) which is provided in IEEE 1588 and is of the type that synchronization is attained by setting time on a packet-by-packet basis are proposed.
FIG. 1 is a diagram illustrating an example of a configuration of a network which is configured to attain clock-synchronization by the Sync-E method. In the above mentioned clock-synchronization, a master clock 10, for example, such as, a GPS (Global Positioning System) clock is supplied to a node N1 and the node N1 synchronizes a synchronous signal (such as a 2-MHz or 2-Mbps signal) used to generate a clock with the master clock 10. Each of nodes N2 and N4 arranged adjacent to the node N1 extracts a clock from a transmission line through which it is connected with the node N1 to attain synchronization. In realizing synchronization in the above mentioned manner, each of the nodes N2 and N4 sends clock quality information to another node and receives the clock quality information from another node using an ESMC (Ethernet Synchronization Messaging Channel) packet for transmission of the clock quality. Likewise, a node N3 extracts clocks from transmission lines through which it is connected with the adjacent nodes N2 and N4. In the above mentioned case, the node N3 selects one of the clocks sent from the node N2 and the node N4 in accordance with the grade of priority which is determined by taking the clock quality into consideration and defines the selected clock as a device clock.
FIG. 2 is a block diagram illustrating an example of a Sync-E correspondence node device (a node device configured to attain synchronization by the Sync-E method). Each of receiving interfaces 11a to 11c extracts a clock and an ESMC packet from a signal which has been received from a transmission line by using a clock recovering function of each PHY section and the reception clocks and the ESMC packets which have been extracted using the respective receiving interfaces 11a to 11c are collected and sent to a clock functioning section 12. An SSM (Synchronization Status Message) which is quality information is extracted from each ESMC packet by using a Sync-E processor 13 included in the clock functioning section 12, the SSM of high quality is selected from within the extracted SSMs by using an SSM selector 14, and a reception clock corresponding to the high-quality SSM is selected from within the reception clocks by using a selector 15. The selected clock is subjected to clock-synchronization using a PLL (Phase locked loop) 16 and is distributed to respective PHY sections of transmitting interfaces 19a to 19c by using a clock distributing section 17. In the above mentioned case, it may become possible to maintain an operating frequency by operating a holdover circuit 18 which is configured to maintain a clock frequency simultaneously with clock distribution even when any clock is not received.
FIG. 3 is a diagram illustrating an example of a configuration of a network which is configured to attain clock-synchronization by the PTP method. In the PTP method, such a time-synchronizing method is adopted that the time on the side of a master is used as a reference and the time on the side of a slave is set to the reference time. A delay in transmission generated between the master and the slave is also calculated and corrected to realize accurate time-based synchronization.
First, a node N11 receives time information from a grand master clock 20 (such as a GPS clock or the like) which is a source of a time with which a time of another node is synchronized in the PTP method to attain time-synchronization. The node N11 sends a PTP packet to each of nodes N12 and N14. As a result, the nodes N12 and N14 are time-synchronized with the node N11. Then, the nodes N12 and N14 send PTP packets to a node N13. The node N13 selects one of the nodes N12 and N14 using a best master clock algorithm (for example, the node N12 is selected). As a result, the node N13 is time-synchronized with the node N12 and time-synchronization of all the nodes is attained.
FIG. 4 is a block diagram illustrating an example of a configuration of a PTP correspondence node device (a node device configured to attain synchronization by the PTP method). Receiving interfaces 21a to 21c respectively detect PTP packets to generate reception times and send the reception time information and the PTP packets to a PTP reception processor 23 which is included in a clock functioning section 22 and performs a protocol analyzing process on the PTP packets. The PTP packets which have been subjected to reception processing using the PTP reception processor 23 are sent to a best master clock algorithm section 24 and the packet of highest quality is selected from within the PTP packets by using the master clock algorithm section 24 and the time of an in-device clock 25 is set to a reception time at which the selected PTP packet has been received. A PTP clock extracting section 27 obtains a difference in time information between PTP packets which are sent thereto at a fixed time and feedbacks information on the obtained difference to the in-device clock 25 included therein to output a clock which is in synchronization with the clock of a source from which the PTP packet concerned has been sent. Then, a PTP packet to be sent is generated by using a PTP transmission processor 26 on the basis of time information from the in-device clock 25 having the time of which has been set in the above mentioned manner. A PTP packet is sent from the PTP transmission processor 26 and is inserted into respective transmitting interfaces 29a to 29c and a time stamp is embedded in the packet as a transmission time.
Incidentally, NTP (Network Time Protocol) which is provided in RFC 1305 and is supplied from a terminal 30 to the in-device clock 25 indicates a time-synchronization method which is performed on the basis of the NTP and is lower in accuracy than the time-synchronization attained the PTP method as a backup time-synchronization method which will be used in the case where utilization of a PTP packet is difficult.
Incidentally, a technique for selectively switching and outputting a plurality of clock signals including clock signals which have been received and extracted using a sending/receiving section and clock signals which have been sent from external clock signal sources using a clock switching section on the basis of quality information which is transferred together with each clock signal is proposed as disclosed, for example, in Japanese Laid-open Patent Publication No. 2000-68966.